Masking for a 2 tone paint job.

Freehand will probably be the best route but I know that can be difficult. Rolled up "worms" of blue tack can give a softer line usually. 

If you are using an airbrush, spray the lighter bottom color first.  I usually mask off the bottom of the wings and elevators after the bottom color is on, then spray the upper color freehand.  Hold the AB at a slight upward angle and spray a light coat on the edge and work up with heavier coats as you get away for th meeting area.  Fairly easy to replicate.  Practice on a milk jug or some scrap, you will get the hang of it in no time.

As an aside, at one point in the war, Navy aircraft on the carriers had a 3 tone paint scheme, white/gray, medium blue and dark blue going from bottom to top.

Yes I saw the three tones, i like that paint scheme. B oth look better than the monotone dark blue.Me freehand? I could mess up a crooked line. I like the blue tack idea.And I like the milk jug idea. I never practice before painting , I just go. And have paid for that.

This is a problem that drives many aircraft modelers to by their first airbrush.  Airbrushes can be adjusted to give a very nice fading line or edge.  And, a double-action brush does the best job of this kind of job.  But, you need to practice!  You need to know just how much to pull the trigger for the width of the fade.

That is the beauty of doing a Hellcat in a two tone.....  all you have to do is run a light coat of blue from the edge of the horizontal stabilizer to the back of the wing and from the front of the wing to the nose, following the curve of the fuselage.  Doesn't have to be exactly straight.  And if you mess up, easy to repaint.  When you get it done, post up a pic, eager to see how it worked out and how you end up doing it.

Hellcats wore three color schemes in USN service during WWII. Initially it was Blue Gray over Light Gray with a wavy feathered demarcation line. Then came the tri color scheme of Sea Blue uppers & Intermediate Blue sides over White lowers. Each color had a fairly straight feathered demarcation line. Then in the final year of the war came the overall Sea Blue scheme...

The result your reffering to is known as a soft edge. On 72nd aircraft i use white tac which i find easier to use than Blue tac.

Another option would to photocoipy the paint instructions and print them out in the correct scale. You can cut them out and lay them on the model on top of thin roll of putty.

You just need anything that will give you a raised edge and be sure to spray from directly above.